This invention relates to a foundry mold for casting a zinc-base alloy, for example, to produce a material-forming die. More particularly, this invention relates to making a mold by sintering a preform of a predominantly alumina particulate mixture that expands during sintering to enlarge a metal-molding cavity to compensate for casting shrinkage during cool down.
In mass production, repetitious forming operations, such as sheet metal stamping or polymer molding, are performed by metal dies. A typical die has a face sized and shaped to fashion a predetermined design in the formed material. Precise die face dimensions are needed to achieve an accurate product design.
Common die-making practice involves precision machining a metal block to sculpture the die face. Such machining is difficult and expensive and substantially prolongs the time required to introduce a product into production. It has been proposed to cast a die so as to substantially form the working face in the casting. A foundry mold is formed from a pattern and defines an open-top casting cavity having a lower surface which bears an impression of the pattern face. Metal is cast into the cavity to produce the die. Finish machining is minimized by a die face that is cast to size, by which is meant that the as-cast face accurately conforms to the desired die face.
A problem in casting to size concerns distortion as a result of shrinkage of the metal during cooling and solidification. This invention is better understood by distinguishing presolidification shrinkage and post-solidification shrinkage. In general, shrinkage that occurs before the metal completely solidifies tends to be nonuniform, i.e., some regions shrink more than others. However, a zinc-base alloy having sufficient durability for die use has been found that is castable with minimal distortion due to presolidification shrinkage. The alloy comprises about 3 to 6 weight percent aluminum, about 5 to 11 weight percent copper and the balance substantially zinc. The alloy is preferably cast in a partially solidified slush form. See U.S. patent application Ser. No. 564,971, filed Oct. 31, 1983, now abandoned.
In contrast, shrinkage of the solidified metal is more uniform. It is a common practice to scale the pattern dimensions greater than the desired die dimensions by a predicted factor to compensate for post-solidification shrinkage. For example, the described zinc-base alloy typically requires a scale-up of about one percent to compensate for post-solidification shrinkage. However, where a model article is available, it is desirable to manufacture the die by constructing a pattern from the article or model without an intermediate scale-up. Similarly, where a model of a die is available, it is desired to use the model as the pattern without resorting to construction of a scaled-up pattern.
It is an object of this invention to provide a method for casting a zinc-base alloy to substantially duplicate a pattern that comprises compacting and sintering a particulate material to form a foundry mold, which material expands during sintering by an amount effective to compensate for post-solidification of the casting. Thus, the product casting accurately conforms to the pattern.
It is a further object of this invention to provide a method for forming a foundry mold for casting to size a die of zinc-base alloy, preferably the described zinc-aluminum-copper alloy, which mold is formed from a pattern that precisely conforms to the desired die working face, but which mold has dimensions larger than the pattern dimensions to compensate for alloy shrinkage. The method comprises compacting a foundry material which is sinterable to produce the mold, but which expands during sintering by an amount corresponding to the post-solidification shrinkage of the alloy. Thus, the cast die after shrinkage accurately replicates the pattern, thereby eliminating the necessity for constructing a scaled-up pattern.